A method of dry-etching a silicon oxide film present on the surface of a semiconductor wafer (hereinafter referred to as a “wafer”) without using plasma in a process of manufacturing a semiconductor device is known. Such a dry-etching method includes a modifying process of generating reaction products by modifying a silicon oxide film by putting a chamber accommodating a wafer into a low pressure state close to a vacuum state and supplying a gas containing at least a hydrogen fluoride gas (HF) into the chamber while adjusting the temperature of the wafer to a predetermined temperature, and a heating process of heating and vaporizing (subliming) the reaction products. This dry-etching method is to etch the silicon oxide film by modifying the surface of the silicon oxide film into the reaction products and heating the reaction products. In the modifying process of the conventional dry-etching method, a mixed gas containing a hydrogen fluoride gas and an ammonia gas (NH3) is supplied to modify the silicon oxide film to produce the reaction products.
The above-described dry-etching method is applied, for example, to a process of etching an oxide film 102 of a wafer W having the structure shown in FIG. 1. As shown in FIG. 1, an interlayer insulating film 101 is formed on the surface of the Si layer 100 of the wafer W. A trench H (for example, a contact hole) is formed in the interlayer insulating film 101, and a silicon oxide film 102 is formed at the bottom of the trench H. A SiN film 103, which is an insulator, is formed on the side wall portion of the trench H.
However, in the conventional dry-etching method, when etching the silicon oxide film 102 formed at the bottom of the trench H of the wafer W, as the processing progresses, the reaction of the silicon oxide film 102 with the mixed gas in the modifying process becomes less active and the amount of modification of the silicon oxide film 102 becomes saturated.
This phenomenon is caused by the reaction products (ammonium fluorosilicate) generated by the reaction between the ammonia gas in the mixed gas and the silicon oxide film 102. As shown in FIG. 2, the reaction products 104 in the trench H are thick in proportion to the modification processing time of the silicon oxide film 102 in the modifying process. When the reaction products 104 are thickly formed in the trench H, the passage speed of the mixed gas decreases, which makes it difficult for the mixed gas to reach the silicon oxide film 102 at the bottom of the trench H. This makes it difficult for the silicon oxide film 102 to be modified at the bottom of the trench H and leaves an unnecessary silicon oxide film 102, even when the reaction products 104 are sublimated in the subsequent heating process.
As described above, although the modification reaction between the silicon oxide film 102 and the mixed gas becomes less active as the process progresses, it is possible to modify all the silicon oxide film 102 at the bottom of the trench H by prolonging the modification processing time. However, when the modification processing time is prolonged, there may be a problem that a portion which is not an etching target is modified, etc.
In addition, in order to sufficiently remove the silicon oxide film 102, it is sometimes necessary to repeat the modification process and the heating process many times. However, when changing from the modification process to the heating process, a process of transferring a wafer to a separate heating chamber is required. Therefore, when the number of times the modification process and the heating process are executed is large, the productivity is lowered.